1. There are two vent systems from each tank. The one we
are most concerned
with I call the "Fill Vent" system. It consists of a fairly large (3/8"
IIRC) hose exiting from the top center of each tank, from the pickup/gauge
unit, right next to the fuel outlet hose going to the engine. This hose MUST
be routed high and steady, with no significant dips, to the steel vent pipe
which runs along the side of the coach next to the filler pipe. (A P.O. has
replaced my tee fitting where the front tank fill vent joins the fore/aft
line with a kluge of brass fittings, but I doubt that's any better than OEM.)
In my coach, that steel pipe is routed just as high as possible through the
frame rails, and is, of course, unclogged. At the front of that pipe, a
large (7/16" or better) hose runs UP to the filler neck, again with no low
spots which might trap fuel.
It is this system which quickly vents the air and vapors while filling the
tanks, and is supposed to spit some gas into the filler neck when the tanks
are full to shut off the nozzle, before compressed air belches a slug of
gasoline back up the fill pipe and all over the ground. Most people with
fill problems (I believe) have a partly clogged or sagged fill vent line,
which obstructs or prevents the venting of the tanks. Remember, the fill
vent line should be level or constantly uphill all the way from the gas tank
to the filler neck. (Some folks have reported that lowering the right rear
suspension while fueling allows them to fill quicker, undoubtedly by
maintaining that uphill slope.)
2. The running vent system vents vapors after the gas cap is
sealed. It occasionally admits air into the tanks to replace withdrawn fuel,
but the vapor pressure of the gasoline ensures this only happens occasionally
in cold weather; the rest of the time the tank is full of almost pure
gasoline vapor. (This is why many fuel-injected cars can put an electric
fuel pump, complete with sparking brushes, right IN the tank, without blowing
up. That TWA 747 that blew up over the Atlantic had an "EMPTY" main fuel tank
which hadn't been filled in months, allowing air to enter with each altitude
change and become explosive.)
Anyhow, if you are finding gas dribbling from the gas cap on the road,
it's likely the regular vent system isn't working. That system connects
fittings on the sides of each tank (2 per tank IIRC) to small (5/16"?) hoses
which lead to the left rear wheel well, hopefully uphill all the way. There,
in the highest spot which is still OUTSIDE the passenger area, they tee
together and enter the fuel separator. The separator is that mysterious
little black box with a plastic (lighter than fuel) marble inside. If more
than a half-cup of liquid fuel makes it this far, the "marble" floats in it,
and blocks off the exit from the separator, preventing liquid gas from
continuing on down the line to the charcoal canister behind/inboard the right
front wheel, where it could contaminate the canister for days, and/or drip on
the ground near the engine causing undue excitement. =8-O The fuel
separator IS available from Cinnabar, but it is unbelievably expensive, so
it's worth trying to clean yours with carburetor cleaner (the marble must
rattle freely when shaken, and you should be able to blow through it when
upright, not when inverted).
Again, sagged, clogged, kinked hoses, a blocked separator, a clogged
charcoal canister (plural in California), even a steel tube blocked with rust
(which Arch reported after diligent sleuthing), can clog this system and
force fuel out the emergency pressure-relief in the cap. And though it
really isn't meant to vent the tanks quickly, this system does allow some
vapor out when fueling, and lets everything settle down so the tanks can be
filled. BTW, once you get 50-odd gallons in, I'd suggest resisting the urge
to top up completely unless you're going out on the highway immediately and
burn some off. The system needs a little "air space" (vapor space) in each
tank to function properly. When all works properly, this is about the time
the automatic nozzle clicks off. (I have gotten around 54 gallons in mine.)
And that's the vent system, to the best of my
simple, but it is spread out over a large horizontal area in the GMC, and
it's hard to see whether a hose is running level, uphill, or downhill, when
lying on your back with road dirt falling in your eyes. Good luck.HTH.Rick Staples
I believe vapor lock is usually caused by high temperature and or altitude (also maybe dirty socks in our tanks)reducing the pressure and causing gas to vaporize in the gas line around the mechanical pump. As the pump / engine sucks harder to supply more gas, the pressure goes down even more until finally there is only vapor and no gas. What the electric pump does is --PUSH-- cool liquid gas to the mechanical pump or direct to the carburetor.
I live in the Mojave desert and have owned GMC's since 11/1977.
Today it hit 110 in my back yard (normal temp).
I have even added temperature sensors to my fuel tanks, then tried hielding from exhaust etc. It didn't matter, the tank temps run 30 degrees above ambient (front tank 5 degrees hotter than the other two).
I finally cured vapor lock by runnung a steel fuel line outside the front frame around to the right front where my fuel pressure sender is. Now I can idle indefinitly in 110 degree weather.
I wrote an article "adding a third fuel tank" that is on the GMC Pacific Cruisers web site (www.gmcpc.org) under "Shadetree". It covers all I have done to eliminate vapor lock. ChuckG
Electric Pump info
I believe there is some urban myth to the hole in the diaphragm
and the collapsed diaphragm on the mechanical pump. Possible but seldom
happens. Many of us have used low pressure electric booster pumps
in front of the mechanical pump for years, and they run all of the
If you are
worried about the mechanical pump replace it or remove it. They are cheap
The electric pump sold by Caspro and others will allow the mechanical pump to draw through it if you do not want to run the electric pump all of the time. The electric pump should be cut into the gas line after the tank selector valve with a filter before the electric pump. The electric pump should be wired through a circuit that stops it from running if the engine stops. This can be either an oil pressure switch or some other circuit like the ones shown on my web page or Kremer's circuits. The filter and pump can be mounted on the cross member just in front of the tanks. This combination has been used for years by the GMC community.
As Dave says, as close to stock as possible. Adding T's , gas lines, etc is adding places for problems to occur.
Dropping the gas tanks is not a hard job. You may want to get the assistance of a second person to keep the tanks steady as you lower, raise them. The tanks are not heavy just awkward.
You will need 4 different sizes of fuel line. They are as
1/2" fuel hose - 3 feet
3/8" fuel hose - 24 feet
5/16" fuel hose - 25 feet
1/4" fuel hose - 10 feet.
You will need 2- 25 foot rolls--$24.75 each for SAE SPEC R7 3/8 line.
You will need about 14 feet of SAE SPEC R7 1/4 inch line@ $.69 a foot for the genset.
20 all SS clamps 20 for $10.50.
I will tell you yes these sound expensive. I do not use the ones from NAPA
or other parts stores. I use the ones for underground sprinkler systems.
The screws on them are also SS not just the band. I get them at
my Truevalue Hardware store.
I did not go with the stock selector valve. I have been quoted some real outrageous prices for them.
I got a generic one from NAPA. #2-2057 price $36.99.
I had to remove the stock bracket and bolt it to the frame. The hose pattern is the same.
Not a big deal for the money saved. Add to that about $6.00 worth of wire ties-----everything is wiretied to the filler line.
The worst part is the vent line that runs to the gas station fill
It runs between the body and the inside molded panel beside the drivers
Sometimes the drain plugs in the bottom of the tanks will not come out. Don't force them, you may damage the tank. Instead, cut the fuel line on the output side of the tanks selector valve. Hook up an electric fuel pump and pump it out. This is easier and you can check to see if the selector valve really works. Don't forget to change the line that connects the generator from the 90 to the electric valve on the backside of the motor. It is hard to get to.
This is what I did and may help others. I searched the web for removal instructions. Finding none I did it this way. Hope this helps. If it is redundant please forgive the posting Fuel Tanks are not that difficult to remove. They are held in place front and rear by two bolts through the straps that run from front to
rear at the rear of the tanks. The tank at the rear overlaps the frame member and does not come down until the front attachment is released. In the front it is held in place by an angle bar that has the straps riveted to it. This angle bar has three bolts securing it to the frame cross member. The rear tank forward bolts can be a little difficult to get into the bolts as the front tank sort of covers them. The secret is to use a floor jack with a 24” X 24” piece of plywood under the center of the tank. It would cover the two fore and aft straps, but that makes not difference as the straps come down with the tank.. Release the front of the tank, three bolts while the tank is supported. Drop the tank a couple of inches to clear the cross member and then release the two rear bolts that hold the rear of the tank. The tank can then be moved slightly forward to clear the rear cross member and then be lowered to the ground. Very easy on a hard surface, but problems on gravel or dirt. The use of plywood in those situation would be of some help. Bruce & Elizabeth Tara F1721
I put off doing it for a couple of years and finally did it my driveway last week. The four rear wheels were up on double planks - about four inches. I then jacked the left side up another four or five inches. With only about two or three gallons left in each tank I lowered them using a small floor jack with a 2'x2' piece of plywood bolted to it. The front tank came out the left side and the rear tank out the the back. Putting them back in was a lot easier than taking them out - for one thing they were empty!
My reason for removing the tanks was to replace all the hoses and rubber fill lines as well as checking the tanks for rust, dirt, etc. inside. The sending unit filters or strainers needed looking at as well. It took over 50 feet of 3/8", 5/16", & 1/4" to put new hoses just to the fuel selector. That includes changing all the hose to the fuel separator behind the left front tire as well and to the Onan. I could not buy locally the replacement strainers or the 1 1/16" dia. fill tube to the front tank and the elbow to the rear tank. Those four pieces had to come from Cinnabar. That meant a two day delay waiting for parts. That's not bad but if I'd known I would have ordered them before I started the job. Al
Cinnabar sells the filter socks separately for about
I ordered all the filler pipe fittings from Cinnabar.
1) 717952 elbow $15.50
1) 717954 hose $ 8.50
1) 717979 elbow $18.70
Not cheap, but not TOO painful. HTH.
These are the folks I used to do my tanks. The only
is that not all of the locations listed on the WEB site have the
large ovens needed for our size tanks. I used Battle
Creek, MI location and was very pleased with the outcome!!
Nice folks - $200 per tank with a lifetime warranty!!
Our tanks are right at the UPS limit of 110 inches total for H + W + D. They were 7in H + 57in W + 46in D. Weight was 46 lbs each without senders. It was the 57in number that is the challenge.
My tanks were horrible. There was almost 1/2 inch of sludge on the bottom and were rusted up
real good. I took mine to Gas Tank RENU. They cut holes in the tops and sandblast and clean the tanks. They then ceramic coat the interior and rubber coat the outside. I have not one problem with stuff cloging my fuel system. Here is a link:
They have dealers all over the country. Here are some pics of mine after they were done.
Thats what I did.Arch
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